This invention concerns devices for the standardization of signal intensity in the magnetic resonance imaging of the human or animal body.
Magnetic resonance images (MRI) of the human or animal body, particularly of the brain, with or without the injection of a suitable contrast medium, is an important diagnostic technique. MRI requires positioning the patient, entirely or in part, inside a strong magnet which creates a homogeneous static magnetic field. With the help of coils, weak magnetic field gradients are superimposed on the static field and radio frequency, electromagnetic impulse fields are applied.
A number of detector coils receive the electromagnetic signals, which are processed to represent raw digital signal intensities (SIr), expressed in arbitrary units, of an anatomic section or volume, described by means of pixel or voxel matrices respectively.
Very often, the raw digital signal intensity (SIr) represents the base for making quantitative evaluations concerning, for example, the increase of the signal intensity in a certain region after administering a contrast medium for MRI, or comparisons of different patients, or between signals received from the same patient at different times, when the identical signal acquisition parameters are kept identical.
Unfortunately SIr is greatly influenced by performance variations with time, which are typical of the MRI machine. These variations, although of modest proportions, must be corrected, or normalized, to make it possible to obtain valid quantitative comparisons between signal intensities monitored at different times.
This correction may be obtained by situating an appropriate standardized source of signal intensity in the immediate vicinity of the patient's body, within the section or volume whose image is to be obtained (this source could be, for example, an aqueous solution of 1 mM nickel chloride), and comparing the SIr values of the image with the signal intensity of the standard (SIst). As a result, the correct, or normalized signal intensity, (SIn) can be expressed using the following formula: ##EQU1##
Similarly, the normalized values of the signal increase due to administration of a contrast medium can, for example, be obtained by measuring the ratios of the normalized signal intensities (SIn) of the region concerned, calculated before and after administration of the above medium, whereas the SIn values are calculated as described above.
Different normalization algorithms may also be used, but in fact all of them give comparable results. To give just one example, it is possible to normalize the signal intensity by simple proportion, assigning a prefixed numeric value to the standard,
The normalization of the signal intensity above described, is highly desirable, if not absolutely indispensable, to improve and make reproducible the comparisons between different patients, or even before and after administration of the contrast medium to the same patient at routine diagnostic procedure level. This in fact is indispensable if computerized--or computer-assisted diagnoses--based on digital imaging are to have a future.
In order to generate an image, the SIr values must be mapped on a discrete scale of greys. The mapping parameters are chosen manually or automatically to obtain the maximum dispersion of gray levels in the anatomic areas concerned. This produces images of the same corporeal regions in different patients only roughly comparable, but does not allow rigourous quantitative comparisons to be made.
The improvement in comparability of the images produced on the basis of a discrete gray scale may be achieved by first normalizing the SIr signal intensity with the help of a standard, as described above, and then imposing a correlation between two values chosen on the scale of the corrected digital signal intensities and two corresponding values on the scale of greys. Preferably, the first corresponding values are chosen inside the lower 40% range of the observed values while the second pair of corresponding values is chosen inside the higher 40% range. These values can be selected quite easily by a software supplied with the imager, which automatically maps the scale of signal intensities observed on the gray scale.
From the point of view of imaging only, it is better that the standard is placed as close as possible to the object being examined, because in this way the lack of homogeneity in the radio frequency electromagnetic field close to the coils is minimized.
The standard can be made up with a sufficiently large volume of a protic liquid, having well-defined longitudinal and transversal protonic magnetic relaxation speeds.
In order to obtain the required standardization, attempts have already been made to use sealed glass bottles or tubes containing solutions of paramagnetic salts, placed close to the patient, inside the chosen NMR imaging plane. This has not proved to be very practical, as the doctor cannot easily foresee all the planes he needs to take, and changing the position of such a standard involves removing the patient from the magnet--which leads to a considerable waste of time and thus increases the procedure's cost.
In addition, glass is fragile and this means that long tubes suitable for the imaging of the entire body are impractical if not even impossible to use.
Usually, these improvised standards were placed on the table on which the patient lies. In some cases they have been fixed to the patient with adhesive tape, or the patient has been asked to hold the standard in his hand. In the case of magnetic resonance of the head the problem is made even more serious due to the fact that the space available inside the room is extremely limited, and a standard of this type can very easily get too close to the coils of the head, thus invalidating the correctness of the results obtained. Probably the unconventionality and unreliability of such improvisations constituted some of the reasons for which the use of a standard has not been more widely adopted or applied in routine clinical practice.
The objective of the invention is thus to eliminate the inconveniences described above and to provide extremely reliable standards, which may be used simply and safely, without the need for the technicians that use them to be specially trained.
An additional objective of the invention is the realization of simple and economical standards, which can be re-used a large number of times, and can be easily adapted to the part of the human body to be analyzed.
The preferred aspects of the invention are described in the corresponding sub-claims.